1. Field of the Invention
The invention relates to an injection device for alternative alcohol fuels (ethanol, methanol), more particularly for direct injection diesel engines, in order to be able to process alcohol fuels, which do not have good spontaneous ignition qualities, in engines without spark ignition.
2. Description of the Prior Art
When using alternative alcohol fuels for direct injection diesel engines, it is necessary to increase the temperature in the combustion chamber of a direct injection diesel engine with a fuel (diesel fuel) which has good spontaneous ignition qualities during the ignition phase of the alcohol fuel, which does not have good spontaneous ignition qualities, to such a degree that its spontaneous ignition temperature is reached.
It is known that in order to process alcohol fuels, which do not have good spontaneous ignition qualities, in engines without spark ignition, particular measures are necessary. This can be achieved, on the one hand, by adding an ignition accelerator, for example kerobrisol or amyl nitrate, in order to redue the temperature necessary for spontaneous ignition. On the other hand, this can be achieved through measures on the engine, namely by increasing the temperature level towards the end of the compression stroke until it is above the spontaneous ignition temperature.
It emerges from the polytropic curve equation for the temperature increase during compression that the temperature of the charge at the end of the compression stroke is dependent essentially on the temperature when the inlet valve closes and on the height of the compression ratio. In addition to the increase in the compression ratio, therefore, the increase in the intake air temperature is important. The latter can be effected either by heating the intake air or by mixing the same with hot exhaust gases which can be taken either from a separate burner or simply from the exhaust line of the engine. With the majority of diesel engines, owing to the long ignition lag, there will be at the moment of ignition already a relatively large amount of ignitable mixture, which results in sudden combustion with every negative effect on the drive equipment and on noise dissipation. MAN's M process avoids this problem with the aid of the wall addition of the fuel, thereby ensuring that once combustion commences only small amounts of fuel are prepared, and that further combustion proceeds in a controlled manner with the aid of the angular momentum of air. At any rate, it has been shown in this connection that starting and running in pure alcohol operation could be effected only with the aid of a starting aid, for example on the basis of a starting pilot arrangement. If, before the engine is switched off, there is a change-over to diesel fuel, then the later start can take place during pure diesel operation. The injection equipment differs from the conventional multifuel engine by a pump element which is enlarged on account of the relatively low calorific value.
The intake pipe injection of methanol, the diesel fuel-methanol-emulsion method and the direct injection of methanol have been examined as further mixture formation methods for running direct injection diesel engines with alcohol fuels. In the case of intake pipe injection, methanol is added to the air admitted in the intake pipe. Ignition is taken over by the conventionally injected diesel fuel. On the basis of various problems--for example, in the partial load range this process results in spark failures so that the proportion of methanol in this operating range must be greatly reduced--any substitution of diesel fuel by methanol with the aid of intake pipe injection in accordance with the present state of development is possible only to a limited extent. Similar problems also arose with the emulsion methods wherein a mixture of diesel fuel and methanol was used in place of diesel fuel. As, at present, a method of producing a stable mixture between diesel fuel and methanol is still not known, an emulsion must be produced in the fuel system by constant, intensive, mechanical mixing. Due to the great vaporization heat, the emulsified methanol has a disadvantageous effect on the ignition lag of the diesel fuel, and increased HC and CO emissions result in the partial load range, and spark failures may even occur.
The direct injection of methanol in conjunction with a diesel fuel injection jet, the so-called ignition jet process, provides better possibilities here. Tests with direct injection of methanol were carried out for two different direct injection processes. These are combustions processes, which are known in practice, having the designations D and Z. The mixture formation is assisted in both combustion processes by a swirl of air produced in the intake channel. The D process works with injection, close to the wall, into a deep trough; a heavy stratification of charge in the form of a ring of mixture is produced in conjunction with a great angular momentum of air. In dual-fuel operation, the methanol is injected close to the wall with the two-hole nozzle conventional for pure diesel operation. In order to start combustion, a diesel fuel ignition jet is introduced into the combustion chamber by means of an additional one-hole nozzle. The direction of the ignition jet is chosen so that the diesel fuel ignition jet serves over the entire speed range as reliable igniter for the methanol air mixture. In the case of the Z process, the fuel is sprayed in standard diesel operation through a centrally arranged four-hole nozzle in a spacially distributed manner into an open, flat combustion chamber trough. In dual-fuel operation with methanol, reliable ignition is achieved with the aid of a diesel fuel injection nozzle which sprays diagonally through the combustion chamber and which has a relatively great free jet length.